Altitude Sickness

Using performance charts in preflight planning should be a familiar routine, but the introduction of computer and electronic flight bag (EFB)-based tools means we are typically just getting a set of key figures, reducing our understanding of where the various limits might sit and what might be the limiting factor.

On a flight from Southern California to New Mexico in the late summer, we needed to climb as the terrain gradually increased from zero feet above sea level (ASL) to 5,000 ft. (1,525 meters) ASL. The temperature at takeoff was 39 degrees Celsius (102 degrees Fahrenheit), dropping to 30 C (86 F) on landing. 

In the cruise at maximum range speed, we noted that we were engine limited but we didn’t think about what that meant for landing since we were going to be much lighter after fuel burn and intended to do a running landing anyway. But as we pulled in the collective lever to cushion the rolling landing, we were still engine limited, making the landing firmer than intended.

We realized that engine limits can be more important than torque when flying in the summer months at altitude. With a gas turbine engine system, the influence of ambient temperature and altitude conditions on the power available change according to which part of the envelope you are sitting in, which means you need to shift your focus on the real limiting factor. While first limit indicator systems on more modern types can help, they don’t indicate where the rate of change in power available shifts.

As the air gets thinner and less dense, the main and tail rotors have to work harder to produce the same lift, meaning the higher you go, the less rotor system performance you get. And instead of gas turbine performance getting better with height, it gets worse. Increases in ambient temperature have a more significant impact on the performance of a gas turbine engine and, as the temperature rises, several key parameters are affected:

1. Air density: Warmer air is less dense than cooler air. This reduction in air density results in less mass flow through the engine, which directly impacts the engine’s power output and efficiency.
2. Compressor performance: The compressor is responsible for increasing the pressure of the incoming air before it enters the combustion chamber. As the air density decreases with rising temperature, the compressor’s pressure ratio and efficiency are reduced, leading to a lower overall rise in pressure across the compressor stage(s).
3. Turbine performance: The turbine extracts energy from the hot, high-pressure gases exiting the combustion chamber to drive the compressor and the helicopters gearboxes to give rotor thrust. Increased air temperature reduces the density and mass flow of the turbine inlet gases, resulting in a decrease in the turbine’s power output.
4. Fuel efficiency: The reduced air density and compressor performance at higher temperatures lead to an increase in the engine’s specific fuel consumption (SFC), meaning more fuel is required to produce the same amount of power or thrust.

In addition to these ambient temperature effects, the altitude that the gas turbine engine operates also impacts its performance:

1. Air density: As altitude increases, the air density decreases dramatically (nominally 30ft/mb). This reduction in air density has the same effect as increasing the ambient temperature, leading to decreases in mass flow, compressor and turbine performance, and overall engine efficiency.
2. Inlet air pressure: The decrease in air pressure at higher altitudes directly affects the engine’s operating parameters. Lower inlet air pressure results in a reduced pressure ratio across the compressor, which in turn, lowers the turbine inlet temperature and overall engine power output.
3. Engine efficiency: The combined effects of reduced air density and pressure at higher altitudes lead to a decrease in the engine’s overall thermal efficiency, as the engine is unable to extract as much energy from the same amount of fuel.

If you look at your weight altitude temperature (WAT) chart for maximum takeoff weight, you can see how these factors start to override the overall helicopter performance by a kink in the chart. At a particular height and temperature, the slope of the graph flattens, meaning that you suddenly have a much larger reduction in takeoff mass for a specific height change. This is the engine limiting conditions coming into play.

As a result, you will need to shift your focus away from the torque gauge and toward the engine gauges because you are going to be limited by gas turbine speed or turbine outlet temperature. In aircraft with automatic limiting systems, this means that you will not be able to pull any more power and your rotor speed will drop.

While you cannot counter this reduction, you can prepare for it by ensuring you understand where the engine power reduction is going to start to take effect. You might get to see it on a simulator checkride, but more importantly, you need to look at WAT charts on a regular basis, noting where the “kink” is when flying in the summer months at altitude.